Combination trap pump



NOV. 8, 1966 ER ETAL 3,283,479

COMBINATION TRAP PUMP I Filed July 9, 1964 2 Sheets-Sheet l INVENTORSTHOMAS H. BATZER ROBERT H. MCFARLAND ATTORNEY Nov. 8, 1966 ER ET AL3,283,479

COMBINATION TRAP PUMP Filed July 9, 1964 2 Sheets-Sheet 2 INVENTORSTHOMAS H. BATZER ROBERT H. MCFARLAND ATTORNEY United States Patent3,283,479 COMBINATION TRAP PUMP Thomas H. Batzer and Robert H.McFarland, Livermore, Califi, assignors to The United States of Americaas represented by the United States Atomic Energy Commission Filed July9, 1964, Ser. No. 381,603 13 Claims. (Cl. 55316) The invention describedherein was made in the course of, or under, Contract W-7405ENG48 withthe United States Atomic Energy Commission.

This invention relates to traps and pumps as utilized in creating veryhigh vacuums. More particularly, it relates to a combination cold trapand sorption pump which has come to be known by the term trump.

The creation of vacuum in the range of to 10- Torr necessitated byrecent advances in various technologies involves requirements that aremuch more exacting a vacuum of lesser degree as employed previously. Thepumps and associated machinery of such a vacuum producing system mustmeet high standards of performance and they must be so designed thatcontaminants do not reach the vacuum chamber. The system must be capableof continuous operation without violation of the integrity of the vacuumwithin the system. The process for producing the vacuum and themachinery employed must be simple to reduce the possibilities formechanical failure.

One of the most universal methods of obtaining high vacuums involves theuse a diffusion pump. A pump of this type does not have the ability toremove gas molecules from the vacuum system by itself, but concentratesthem for removal by a second pump. This second pump, called a backingpump, is usually of the mechanical or sorption variety, .both of whichhave certain disadvantages which will hereinafter become apparent.

A basic problem in producing ultra-high vacuum is the occurrence oflarge pressure fluctuations called bursts. It is thought that thebursts, which register as fast rising excursions of the pressure gauges,arise from mechanical pump oil molecules within the diffusion pumpboiler cracking into lighter fractions due to the high boilertemperature and low thermal stability of the oil molecules. Thispresupposes that the system is tight. Pressure bursts sometimes appearwhen O-ring seals are employed. It is further theorized that thesemechanical pump oil molecules invade the diffusion pump boiler bybackstreaming from mechanical forepressure pumps used to back thediffusion pump.

One conventional way of eliminating pressure bursts is to employ twodiffusion pumps in series backed by a mechanical pump. Then the bulk ofthe back-streaming mechanical oil cracks in the intermediate stagebacking diffusion pump so that the primary diffusion pump, working onthe vacuum chamber, only encounters the lighter cracked oil fractions.These cracked fractions are in the form of non-condensible gases, notoil vapor. Therefore, no further cracking occurs and the gas cannotpenetrate the primary diffusion pump jets. Other bulky multicomponentcomplicated apparatus arrangements have also been utilized in attemptsto eliminate bursts. However, there exists a pressing need for a compactsimplified apparatus to obtain high vacuum reliably Without bursts.

The present invention provides a combination trap and pump, or trump ina form which eliminates offending and unnecessary machinery from .avacuum pumping system. Such a trap-pump or trump is comprised of agastight outer casing provided with gas inlet and outlet means which arespatially intercommunicated by the volume within the interior of saidcasing so that gas can pass Patented Nov. 8, 1966 therebetween. A largegas sorptive capacity means is disposed in the volume for contacting gaspassing therethrough. This sorptive means has the capacity for sorbinglarge quantities of gas in relation to the volume of the outer casing.An oil molecule sorptive metal surface means is also disposed in thevolume to intercept oil molecules entering said inlet means and is inspatial intercommunication with said gas sorptive means. Thiscombination trap-pump provides a new and valuable device which solvesmany of the problems encountered heretofore in producing high vacuums.For example, it is a simple, eifective, inexpensive device virtuallyfree of sources of possible mechanical failure. It can be regeneratedwithout removal from the pumping system and performs very eflfectivelyin either capacity as a trap 'and/ or a pump. It has no moving parts towear out and is also noiseless and vibrationless, very unlike themechanical pumps it replaces. eliminates the need for both themechanical backing pump and the intermediate stage diffusion pump,thereby reducing the necessary machinery from two expensive complicatedvacuum pumps to one relatively inexpensive simple reliable machine. Inaddition, the trump can be used by itself to effect high vacuum wheresmaller vacuum chambers are involved.

A conventional sorption pump can be utilizedto back a diffusion pump,but the problem there is that they cannot be pumped through, i.e., intoand out of, and they must be degassed at atmospheric pressure. Thepresent invention is also a sorption pump, but one which can beregenerated conveniently in working position, at low pressure, unlikemost sorption pumps, and in addition, performs effectively as a trap toprevent backstreaming mechanical pump oil molecules from penetratingfurther in the vacuum pumping system.

Accordingly, it is an object of the present invention to provide asorption pump which will independently back a difiusion pump.

It is another object of the invention to provide an efficient sorptionpump which can be regenerated without removing it from a pumping system.

It is afurther object of the invention to provide a sorption pump whichalso functions effectively as a trap.

It is yet another object of the invention to provide an eflicientcombination trap-pump of simple yet effective design to prevent thepossibility of contaminating the vacu- It is still a further object ofthe invention to provide a combination trap-pump, economical in cost,which will perform effectively for extended periods of time withoutfailure.

A general advance in the art and other objects will appear hereinafterare also contemplated.

The combination trap-pump will be described with reference to theaccompanying drawings, of which:

FIGURE 1 is asectional view of a generic first embodiment;

FIGURE 2 is a sectional view of a second and preferred embodiment; and

FIGURES 3 and 4 are schematic diagrams of the device of the inventionarranged in a pumping system.

Referring to FIGURE l, shown therein is the simplified, although highlysuccessful, working embodiment 10 of the trap-pump of the invention,also termed trump hereinafter. An external or outer casing 11 isprovided as a cylindrical tube. This configuration was chosen hecause ofits structural rigidity and ability to withstand pressure when theinterior is evacuated. Any configuration, preferably self-sustaining, ofouter casing w il-l sudiice so long as it is gas tight and willwithstand atmospheric pressure and is constructed of vacuum servicematerial The present invention adapted for immersion in cryogenic.coolant. Type 304 stainless steel has successfully been used for theouter casing, but any material could be employed which is not porous,cleans up easily, does not out-gas significantly, and can withstandextreme temperature cycling. End cover 12 is mated to the outer casingwith a vacuumtight seal and has tubes 13 and 14 projecting therethroughin sealed relation. Tubes 13, 14 are open at both ends and the exteriorends which terminate outside the trump casing are formed for coupling bymeans of vacuum tight seals to a vacuum pumping system. The interior endof tube 13 extends into the trump casing 11 to approach vacuum tight endcover 16 sealing the other end of easing l 11. The interior end of thetube 13 is out (man angle for purposes of assembly. Tube 14 extends onlyshortly into the trump casing past the end cover 12. These differentlengths of inlet and outlet tubes expose the gas to the greatest portionof the contents of the trump by forcing it to traverse, in effect, fromone end of the casing i to the other. End cover 16 is removable forinitial packing and .to enable the contents of the trump to be easily Ireplaced. 1 Inside the outer casing 11 a tightly packed column of copperturnings 17, commonly called copper wool, extends along an arcuate orcircuitous path from inside the internally projecting end of tube 14 toend cover 16 and then into. the internally projecting end of tube 13.:A-lso, a cross layer of copper wool 18 is located inside the outercasing 11 approximately midway betweenthe lend covers 12 landl6 toprovide a thermal conductivity :path which spans the cross section. Thisthermal path serves to distribute and reduce thermal gradients within 1the trump and aids in reducing the cool down and bakeout times. 1 Otherwool or tunings 17 than copper could be used ;such as stainlesssteel,bronze, aluminum, etc., but copper is used with preference sincein aclean condition it has nan unusual or high aflinity for oil molecules.One of ithe, main values of the present invention is its ability to,trap backstreaming oil vapor from mechanical vacuum pumps by using alarge surface area of clean copper to capture the oil molecules. Thelocation of the trump in the pumping system, and its internal design,ensures conitact between the ba-ckstreaming oil molecules and the ,cleancopper. This causes the effective trapping of oil molecules, and yetpermits, and does not hinder, reverse flow of the :gas molecules beingevacuated. Molecular isieve material 19, in bead form, is disposed inthe remaining space within casing 11. The copper wool column 17 3 packedtightly into the ends of the tubes 13, 14 prevents jthe beads ofmolecular sieve material 19 from entering therein. The copper wool alsokeeps the sieve material from being blown out of the trump through thetubes 13, 5214 if pressure is quickly let into the trump. The copperwool 17, in addition to trapping backstreaming oil vapor from themechanical vacuum pump, provides for conduct- ;ance of heat to and fromthe sieves 19 since the con- 'ductance' of the copper wool is greaterthan that of ,the sieve material.

Molecular sieve material 19, sometimes called artificial fzeolitepellets or beads, which fills the remaining volume lot the trump, i.e.,from end cover 12 to the copper wool layer 18 and from the copper wollayer 18 to the copper wool adjacent end cover 16, is in communicationwith the I ;!gas stream in the. copper wool. 7 l The molecular sievematerial or sieves which are the "commercial product of the UnionCarbide Corporation marketed under the Linde trademark are suitable foripresent purposes; Such sieve materials are alkali metalaluminosilicates quite similar to many natural clays and feldspars.Three types are available: 4A, 5A, and 13X. The generalformula for type4A is 0.96:0.04 Na O- 1.00 fAl O -l.92i0.09 SiO -H O. TypeSA is producedthorn ,type 4A through ion exchange of about 75% of the sodium ions forcalcium ions. These sieves are a new series of commercial absorbents.The crystals are highly .porous with pores only 16-20 billionths of aninchin diameter and of uniform size. Other materials which couldsatisfactorily be substituted for the zeolite are activated alumina andactivated charcoal, the latter of the two being particularly effective.

The whole trump of FIGURE 1 is immersed in a cryogenic liquid,preferably liquid nitrogen during operation as either a trap or a pump.The low'temperat'ure of the liquid increases the gas sorbing ability ofthe molecular sieves.

Utilizing a trump, instead of a pump of the sorption trump, instead of amechanical pump as a backing pump for a diffusion pump, is that a trumpeleminates the possibility of backstreaming -oil vapor, an undesirablecharacteristic which often accompanies mechanical pumping. Preferredembodiment 20 of the invention is depicted in FIGURE 2 to whichreference is now made..

A central stainless steel tube 21 is used therein cforcontaining acryogenic liquidfor cooling the molecular sieves. Thetube is made ofrelatively thin wall construction to reduce consumption of the coolingliquid.

Accordion structure ribs 22 provide structural strength to withstand'the pressure difierentials involved due to the thin wall construction.An alternative embodiment could have the cryogenic liquid coaxially'surrounding .the

molecular sieves and copper wool with the gas flow through the center,but that arrangement would be less efiicient in the use of the cryogenic=liquid. A copper sheath 23 depends from and coaxially surrounds. the.

central tube in spaced relation forming an annular .void between the.tube 21 and the sheath. The sheath is1attached to or in contact withthe, central tube 21 as a.

heat transfer to the sheath 23. The sheath has a large number ofperforations 26 through it to permit the passage of gas flowtherethrough. A copper screen 27. lines the inside of the sheath 23 toconstrain molecular :sieve. material 28 which fills theannular voidbetween the central tube 21 and the sheath 23.

A polished copper skirt 29 depends from the central tube 21jtrom a hardsolder joint 31 and coaxial ly surrounds the inner sheath 23 in spacedrelation thereto,

thereby permitting the free passage of gas flow up into the skirt 29 tocontact the sieves through the holes 26. The 'pohshed copper skirt 29provides a heat shield reducing the heat load from Warm casing wall 32vto the zeolite bed.

A re-entrant truncated conical copper partition 33 seals the lower endof the central tube 21. A copper truncated cone shaped skirt 34 forms anose on outlet. tube; 35

and is disposed in parallel relation to the re-entrant copper partition33 by copper spacing and heat transfer ribs 37 which are hard solderedto the reentrant partition 33.

Copper wool 38 fills the space between the remnant partition 33,-theflanged end 34, and the ribs 37. The a copper wool eflects a coldcondensing surface which in-' tercepts backstreaming oil vapor enteringthe trumpv through the outlet tube 35 which as usually operated connectswith a mechanical fore-pressure pump. The whole unit is enclosed withina gas tight stainless steel'outer casing 32. One end of the casingmay-be sealed as at 39 to a flanged removable cover, e.'g., to a flangeof a seal 41 as described in US. patent application Serial No.

236,750, filed December '1, 1962, by Thomas H. =Batzer,:

nection of the trump 20 with :a vacuum chamber or a diflusion pump.

'I hree modes of operation employing apparatus of the present inventionare contemplated. The following explanation is made with reference toFIGURE 3. Therein is shown a simple vacuum pumping system utilizing atrump. A vacuum chamber is connected to the trump through valve B, andthe trump is connected to a mechanical pump through valve A.

In the first mode of operation the trump is closed off from the vacuumchamber by valve B. Then the trump is cleaned up by simultaneousdegassing and baking out. This involves opening valve A and using themechanical pump to evacuate the trump while simultaneously heating it ataround 350 C. During cleanup, the trump traps backstreaming oilmolecules from the mechanical pump. When the trump reaches about 10microns or less of pressure at approximately 350 C., the mechanical pumpused during cleaning is valved off by closing valve A. The trump is thenfilled with a cryogenic liquid (the embodiment 10 show by FIGURE 1 isimmersed in the cryogenic liquid). The trump is then valved open to thevacuum chamber by opening valve B, and it then pump the gases out of thechamber :by sorption. This mode is usually employed on smaller volumes,in comparison with the trump capacity. The trump in this mode ofoperation and under these conditions will, by itself, create .a vacuumof about 0.1 micron, but usually the evacuation process is stopped ataround 5 or 6 microns. Then the mechanical pump is started and valved inby opening valve A to remove the remaining inert gases, e.-g., helium,through the trump. During this operation the trump acts as a trap andcaptures backstreaming oil vapor from the mechanical pump. Utilizingthis first mode of operation for the trump provides very clean vacuumsand produces excellent results such as pressures in the range of l ofmercury.

It should be pointed out that it is not necessary to fill the trump withliquid nitrogen for it to operate as a pump. After bake out themechanical pump could be valved off and the trump opened to the volumeto be evacuate-d. During cool down to room temperature, the will pump ona vacuum chamber, and it can be made to pump further by filling it firstwith water and then with ice Water, or further still by packing it withDry Ice, or yet further still by successively cooling the trump tofurther lower temperatures with various cryogenic liquids. *It istherefore not necessary to have liquid nitrogen available to evacuate avolume to moderate vacuums with the trump.

In the second mode of operation the trump is first cleaned up as in theprevious modeof operation. The mechanical pump is valved off by closingvalve A and the trump is filled with liquid nitrogen. The trump is thenvalve open thruogh valve B to the vacuum chamber and used to rough out,or partially evacuate the chamber. Then the trump is valved oil" fromthe chamber by closing valve B and it is cleaned up as before by asecond bake out and evacuation. The mechanical pump is then valved offby closing valve A and the trump filled with liquid nitrogen and againopened to the vacuum chamber through valve B. The diffusion pump isstarted and the trump then acts as a backing pump for the diffusionpump. This is for larger volume evacuations as compared with thecapacity of the trump. Using this mode of operation guarantees nob'ackstreaming mechanical pump oil vapor, but it does not necessarilycreate lower base pressures than obtainable by the third mode ofoperation.

The third mode of operation is the fastest way to produce very cleanultra-high vacuums and is the method most usually employed. It workswell with all size volume vacuum chambers. Valve B is closed and valve Ais opened. The trump is evacuated up to valve B with the mechanical pumpand then cooled with liquid nitrogen. The vacuum chamber is up tostandard air pressure. Valve B is then opened and the mechanical pump isoperated to rough the system down to approximately 1 micron, while thetrump trap backstreaming oil vapor from the mechanical pump. After thesystem is roughed, valve B between the trump and the vacuum chamber isclosed. The trump is then cleaned up by bake out and evacuation, asdescribed before. The mechanical pump is then valved olf by valve A, thetrump is cooled with liquid nitrogen, the diffusion pump is started, andthe trump is valved in, by opening B, to back the dilfusion pump andpumping then proceeds as in the second mode.

These three modes of operation provide an extremely clean system andeliminate the mechanical vacuum pump from the ultrahigh vacuum pumpingto provide a smooth vacuum pressure without pressure derivation. Bybacking properly trapped dilfusion pumps with trumps, base pressures of2 l0 mm. of mercury have been produced. Before the development of thetrump these results were very difiicult to achieve and more difficult tomaintain. The vtrump is responsible for simplifying the method ofproducing ultra-high vacuums, and making it less susceptible tomechanical failure.

An enlightened employment for utilizing the trump places two trumps inparallel between the mechanical forepressure vacuum pump and thediffusion pump in the vacuum chamber. FIGURE 4 presents this arrangementschematically. Thus, while one trump is backing the difiusion pump, theother can be undergoing clean up by evacuation and bake out. This is thepreferred embodiment in modes 2 and 3. A faster method of evacuation isthereby provided and a way to maintain an ultra-high vacuum indefinitelyis achieved.

Although several embodiments of the invention are possible, andalternatives have been described, these are merely illustrative andvarious modifications can be made Without departing from the spirit andscope thereof. It is to be understood, however, that this invention isnot limited to the specific embodimens thereof except as encompassed inthe following claims.

What is claimed is:

1. A combination trap-pump for producing vacuums comprising;

(a) a gas tight outer casing having inlet and outlet means spatiallyintercommunicated by the volume within said casing for passage of gastherebetween,

(b) means providing a large gas sorptive capacity diposed in said volumefor contacting gas passing therethrough, and

(c) previous oil molecule sorptive metal surface means disposed in saidcasing to competely transect the gas passage volume therein in at leastthe oulet means region thereof for interception of oil moleculesentering from said outlet means and being in gas passageintercommunication with said gas sorptive means of paragraph (b).

2. The trap-pump of claim 1 wherein the gas sorptive means is selectedfrom the class consisting of artificial zeolites, activated charcoal,and activated alumina.

3. The trap-pump of claim 1 wherein the metal sorptive means is selectedfrom the class consisting of copper, bronze, stainles steel, andaluminum.

4. The trap-pump of claim 1 wherein;

(a) the gas sorptive means comprises artificial molecular sievematerial, and

(b) the sorptive metal surface means comprises clean copper surfaces.

5. A combination trap and pump for producing vacuums comprising;

(a) a gas tight cute-r casing capable of withstanding extremetemperature cycling,

(b) at least first and second open tubes projecting through andterminating inside said outer casing, said tubes comprising means forintroducing and exhausting gas from said casing,

(c) copper wool forming a path inside said outer casing between theinterior terminations of said first and sec ond tubes, and

(d) molecular sieve material filling the remaining volume of said outercasing.

6. A combination trap-pump for producing vacuums omprising;

(a) a tubular gas tight outer casing capable of withstanding extremetemperature cycles,

(b) first and second end covers for the ends of said casing,

(c) a first open tube projecting in sealed relation through the first ofsaid end covers and terminating inside said outer casing near the secondof said end covers,

((1) a second open tube projecting in sealed relation through the firstof said end covers into said outer casing and terminating near saidfirst end cover,

(e) copper wool disposed inside said outer casing forming a pathextending from inside the end of said first tube to terminate inside theend of said second tube,

(f) a layer of copper wool extending transversely across the interiorcross section of said outer casing, and

(g) molecular sieve material filling the remaining volume within saidouter casing.

7. A combination trap and pump for producing vacuums comprising;

(a) a gas tight outer casing having inlet and outle means spatiallyintercommunicated by the volume within said casing defining a gas flowpassage therebetween and having a chamber for containing a coolantdefined by a re-entrant receptable wall portion projecting within saidvolume,

(b) particulate gas sorptive material arranged in heat exchange relationwith said receptacle wall portion and in gas flow communication withsaid gas flow passage Within said casing, and

(c) pervious sorptive metal surface means disposed in said casing tocompletely transect said gas flow passage in a region between saidoutlet means and said particulate gas sorptive means.

8. The trap and pump of claim 7 wherein (a) the gas sorptive meanscomprises molecular sieves,

and

(b) the sorptive metal surface means comprises clean copper.

9. A combination trap and pump for producing vacums comprising (a). agas tight outer casing having inlet and outlet means spatiallyintercommunicated by the volume within said casing to define a gas flowpassage therebetween and provided with a re-entrant partition defining acentral chamber for holding a cryogenic liquid,

(b) molecular sieve material surrounding said partition in thermalcommunication with said central chamber and gas flow communication withsaid gas flow passage in said casing,

(c) means for containing said molecular sieve in thermal communicationaround said central chamber and for permitting the passage of gasmolecules therethrough, and

(d). clean pervious copper sorptive surface means disposed in saidcasing to completely intersect said gas flow passage in a region betweensaid outlet means and said molecular sieve material.

10. The trap and pump of claim 9 wherein (a) said seive containing meansconsists of screen material, and

(b) said clean copper sorptive surface means consists of copper wool.

ums comprising (a) a gas tight outer casing having inlet and oulet meansspatially intercommunicated by the volume within said casing andprovided with a partition de fining a central'chamber for holding acryogenic liquid,

(b) molecular sieve material surrounding and in therma1 communicationWith the said central chamber, (6) copper screen material for containingsaid sieves in thermal communication around said central chamber and forpermitting the passage of gas molecules therethrough,

(d) polished copper sheet contiguous in spaced relation around saidscreen material in the path of gas fiow, and

(e) copper wool surrounding said outlet means dis:

posed in the path of gas flow and in heat exchange relation with saidcentral chamber. 12. A combination trap and pump for producing vacuumscomprising (a) a generally cylindrical cold wall tube having top andbottom ends and partitioned central well portion,

(b) molecular sieve material surrounding said cen-,

tral portion of said tube and constrained therearound by copper screenmaterial, (c) a polished copper skirt depending from said cylindicaltube in spaced relation around said constrained 3 molecular sieve,

(d) it re-entrant partition sealing the bottom end of said tu e,

(e) an oulet tube having a flanged ,end formed in a skirt parallel inspaced relation With;said partition,

(f) copper wool disposed between said re-ent-rant partition and saidskirt of said outlet tube,

(g) a double ended gas tight outer casing having the first end sealed ingas tight relation around the top end of said cold Wall tube said casingenclosing said topper skirt with the second end sealed in gas tightrelation around said outlet tube, and

(h) an inlet tube communicating with the interior of said outer casing.

13. A combination trap and pump for producing .vac-r ums comprising (a)a stainless steel accordioned relatively thin walled cylindrical coldwall partion tube,

(b) a coaxial cylindrical copper sheath depending from and surroundingsaid partition tube in spaced relation thereto and having multipleperforations therethrough and attached to said tube with a thermaltransfer joint,

(c) molecular sieve material, disposed between said sheath and saidpartition tube, (d) copper screen adjacent to and inside said sheath andconstraining said sieve material against said par (i) a double ended gastight stainless steel outer casing having the first end sealed in gastight relation around the upper end of said cold wall partition tube,the other end of said casing sealed ingas tight relation around saidoutlet tube enclosing said copper skirt of said outlet tube, said casingenclosing said copper skirt depending from said partition tube, and

9 10 (j) an inlet tube communicating with the interior of 3,172,7453/1965 Needham et a1. 55 269 said outer casing. 7 3,200,569 8/ 1965Wheeler 55208 References Cited by the Examiner OTHER REFERENCES UNITEDSTATES PATENTS 5 German printed application No. 1,110,356, July 1961.

2,465,229 3/1949 Hipple 55-387 X 2,841,323 7/1958 Lindenblad 230-69REUBEN FRIEDMAN Exammer- 3,081,068 3/ 1963 Milleron 55269 X J. ADEE,Assistant Examiner.

1. A COMBINATION TRAP-PUMP FOR PRODUCING VACUUMS COMPRISING; (A) A GASTIGHT OUTER CASING HAVING INLET AND OUTLET MEANS SPATIALLYINTERCOMMUNICATED BY THE VOLUME WITHIN SAID CASING FOR PASSAGE OF GASTHEREBETWEEN, (B) MEANS PROVIDING A LARGE GAS SORPTIVE CAPACITY DIPOSEDIN SAID VOLUME FOR CONTACTING GAS PASSING THERETHROUGH, AND